Until relatively recently, all Märklin printed material, catalogues, leaflets, instructions and yes, boxes, carried a very helpful printing code which included the date (month & year) of printing. It's been discussed here a number of times.

It's not conclusive proof of the age of the model within, but is generally close enough for most needs and is at least a starting point.

On some boxes, finding the code can be a bit of a challenge if you don't know where to look but usually will be found in the folds of the box end-flaps.

Typically the code is in the format of a 5-digit number relating to the model, then the print-run code (number of items printed), then the date code MM YY, and finally the code for the actual company that printed them.

Here's one example of how it might look (as I said, can take some imagination to find) - in this instance, "2 73" is the key, Feb 1973



Sometimes they are right there on the flap, clear as day